NF4+ WF7- and NF4+ UF7- and methods of preparation

ABSTRACT

Salts of the formula NF 4   +  MF 7   -  are produced by the  fowing reaction 
     
         NF.sub.4 HF.sub.2 nHF+MF.sub.6 →NF.sub.4 MF.sub.7 +(n+1)HF 
    
     wherein M is uranium (U) or tungsten (W).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to energetic inorganic salts and more particularly to salts containing the NF₄ ⁺ cation.

2. Description of the Prior Art

NF₄ ⁺ salts are key ingredients for solid propellant NF₃ -F₂ gas generators, as shown by D. Pilipocich in U.S. Pat. No. 3,963,542, and for high detonation pressure explosives, as shown by K. O. Christe in U.S. Pat. No. 4,207,124. The synthesis of NF₄ ⁺ salts is unusually difficult because the parent molecule NF₅ does not exist and the salts must be prepared from NF₃ which amounts formally to a transfer of F⁺ to NF₃ accordingly to:

    NF.sub.3 +F.sup.+ →NF.sub.4.sup.+

Since fluorine is the most electronegative of all elements, F⁺ cannot be generated by chemical means. This difficult synthetic problem was overcome by K. O. Christe and co-workers, as shown in U.S. Pat. No. 3,503,719. By the use of an activation energy source and a strong volatile Lewis acid, such as AsF₅, the conversion of NF₃ and F₂ to an NF₄ ⁺ salt became possible: ##STR1## However, only few Lewis acids are known which possess sufficient strength and acidity to be effective in this reaction. Therefore, other indirect methods were needed which allowed conversion of the readily accessible NF₄ ⁺ salts into other new salts. Two such methods are presently known. The first one involves the displacement of a weaker Lewis acid by a stronger Lewis acid, as shown by K. O. Christe and C. J. Schack in U.S. Pat. No. 4,172,881 for the system:

    NF.sub.4 BF.sub.4 +PF.sub.5 →NF.sub.4 PF.sub.6 +BF.sub.3

but obviously is again limited to strong Lewis acids. The second method is based on metathesis, i.e., taking advantages of the different solubilities of NF₄ ⁺ salts in solvents such as HF or BrF₅. For example, NF₄ SbF₆ can be converted to NF₄ BF₄ according to: ##STR2## This method has successfully been applied by K. O. Christe and coworkers, as shown in U.S. Pat. Nos. 4,108,965; 4,152,406; and 4,172,884, to the syntheses of several new salts. However, this method is limited to salts which have the necessary solubilities and are stable in the required solvent. The limitations of the above two methods are quite obvious and preempted the syntheses of NF₄ ⁺ salts of anions which are either insoluble in those solvents or are derived from a Lewis acid weaker than the solvent itself and therefore are displaced from their salts by the solvent.

SUMMARY OF THE INVENTION

Accordingly an object of this invention is to provide methods which permit the syntheses of new NF₄ ⁺ salts containing anions derived from very weak Lewis acids.

Another object of this invention is to provide new energetic NF₄ ⁺ compositions which are useful in explosives and solid propellants.

A further object of this invention is to provide NF₄ ⁺ compositions for solid propellant NF₃ -F₂ gas generators for chemical HF-DF lasers which deliver a maximum of NF₃ and F₂ while not producing any gases which deactivate the chemical laser.

Yet another object of this invention is to provide NF₄ fluorotungstates which on burning with tungsten powder can produce hot WF₆ gas in high yield.

These and other objects of this invention are achieved by providing:

Salts of the formula NF₄ ⁺ MF₇ ⁻ by the following reaction

    NF.sub.4 HF.sub.2.nHF+MF.sub.6 ⃡NF.sub.4 MF.sub.7 +(n+1)HF

wherein M is uranium (U) or tungsten (W). These salts are useful as ingredients in solid propellants and in high detonation pressure explosives.

A method of generating hot WF₆ gas by burning a mixture of NF₄ WF₇ and tungsten metal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Surprisingly, it has now been found that the salts NF₄ UF₇ and NF₄ WF₇ can be prepared from the very weak and volatile Lewis acids UF₆ and WF₆. The salts are prepared by the following methods.

First, readily available NF₄ SbF₆ salt is converted by metathesis into NF₄ HF₂ according to the reaction ##STR3## The details of this procedure are disclosed by K. O. Christe, W. W. Wilson, and R. D. Wilson in Inorg. Chem., 19, pp. 1494+(1980), herein incorporated by reference. A method of preparing NF₄ SbF₆ is disclosed by K. O. Christe, C. J. Schack, and R. D. Wilson, J. Fluorine Chem., 8, pp. 541+(1976), herein incorporated by reference.

The NF₄ HF₂ produced by the above procedure will be complexed with HF and can be represented by the formula NF₄ HF₂.nHF. Because HF is a stronger Lewis acid than either UF₆ or WF₆, as much HF as possible has to be removed from the NF₄ HF₂ without decomposing the NF₄ HF₂. This can be achieved by judicious pumping at about 0° C. This is continued until a solid having the composition NF₄ HF₂.nHF wherein n is from about 0.5 to about 10.0 is obtained.

NF₄ UF₇ and NF₄ WF₇ are produced by the following reactions:

    NF.sub.4 HF.sub.2.nHF+UF.sub.6 ⃡NF.sub.4 UF.sub.7 +(n+1)HF (2)

    NF.sub.4 HF.sub.2.nHF+WF.sub.6 ⃡NF.sub.4 WF.sub.7 +(n+1)HF. (3)

These reactions may be run at ambient (25° C.) temperature. Repeated treatments of NF₄ HF₂.nHF with a large excess of UF₆ or WF₆, followed by the removal of the volatile products at ambient temperatures, surprisingly shifted the equilibrium in reaction (2) and the equilibrium in reaction (3) quanitatively to the right. This is probably due to the thermal stability of NF₄ UF₇ and of NF₄ WF₇ being significantly higher than that of NF₄ HF₂.

The addition of UF₆ or WF₆ and subsequent evacuation of volatile reaction products is continued until the conversion of NF₄ HF₂.nHF to NF₄ UF₇ or NF₄ WF₇ is substantially completed. This will be the point at which no significant amount of UF₆ or WF₆ is taken up and no significant amount of volatile reaction products (e.g., HF gas) is generated. Thus, by monitoring the gases evacuated from the reaction chamber, the progress of the reaction may be monitored.

Examples 1 and 2 further illustrate these procedures.

NF₄ UF₇ and NF₄ WF₇ are useful as key ingredients for solid propellant NF₃ -F₂ gas generators and for high detonation pressure explosives.

NF₄ WF₇ is of particular interest as an ingredient for hot WF₆ gas generators. Hot WF₆ is an excellent electron capturing agent and therefore useful for reducing radar signatures. For example, formulations based on

    6NF.sub.4 WF.sub.7 +5W→11WF.sub.6 +3N.sub.2         ( 4)

can theoretically produce up to 97 weight percent of WF₆ with flame temperatures in excess of 2000° C. A pyrotechnic mixture of finely powdered NF₄ WF₇ and tungsten in approxiately a 6:5 molar ratio may be used.

The general nature of the invention having been set forth, the following examples are presented as specific illustrations thereof. It will be understood that the invention is not limited to these examples but is susceptible to various modifications that will be recognized by one of ordinary skill in the art.

EXAMPLE 1 Preparation of NF₄ WF₇

Dry CsF (15.0 mmol) and NF₄ SbF₆ (15.0 mmol) were loaded in the drybox into one half of a prepassivated Teflon double U-metathesis apparatus. Dry HF (15 ml liquid) was added on the vacuum line and the mixture was stirred with a Teflon coated magnetic stirring bar for 15 minutes at 25° C. After cooling the apparatus to -78° C., it was inverted and the NF₄ HF₂ solution was filtered into the other half of the apparatus. Tungsten hexafluoride (22.5 mmol) was condensed at -196° C. onto the NF₄ HF₂. The mixture was warmed to ambient temperature, and two immiscible liquid phases were observed. After vigorous stirring for 30 minutes at 25° C., the lower WF₆ layer dissolved in the upper HF phase. Most of the volatile products were pumped off at ambient temperature until the onset of NF₄ HF₂ decomposition became noticeable (NF₃ evolution). An additional 8.0 mmol of WF₆ was added at -196° C. to the residue. When the mixture was warmed to ambient temperature, a white solid product appeared in the form of a slurry. All material volatile at -31° C. was pumped off for 1 hour and consisted of HF and some NF₃. An additional 14.5 mmol of WF₆ was added to the residue and the resulting mixture was kept at 25° C. for 14 hours. All material volatile at -13° C. was pumped off for 2 hours and consisted of HF and WF₆. The residue was kept at 22° C. for 2.5 days and pumping was resumed at -13° C. for 2.5 hours and at 22° C. for 4 hours. The volatiles collected at -210° C., consisted of some HF and small amounts of NF₃ and WF₆. The white solid residue (5.138 g, 84% yield) was shown by vibrational and ¹⁹ F NMR spectroscopy to consist mainly of NF₄ WF₇ with small amounts of SbF₆ ⁻ as the only detectable impurity. Based on its elemental analysis, the product had the following composition (weight %):

NF₄ WF₇, 98.39; CsSbF₆, 1.61. Anal. Calcd: NF₃, 17.17; W, 44.46; Cs, 0.58; Sb, 0.53. Found. NF₃, 17.13: W, 44.49; Cs, 0.54; Sb, 0.55.

EXAMPLE 2 Preparation of NF₄ UF₇

A solution of NF₄ HF₂ in anhydrous HF was prepared from CsF (14.12 mmol) and NF₄ SbF₆ (14.19 mmol) in the same manner as described for example 1 (NF₄ WF₇). Most of the HF solvent was pumped off on warm up from -78° C. towards ambient temperature, until the onset of NF₄ HF₂ decomposition became noticeable. Uranium hexafluoride (14.59 mmol) was condensed at -196° C. into the reactor, and the mixture was stirred at 25° C. for 20 hours. The material volatile at 25° was briefly pumped off and separated by fractional condensation through traps kept at -78°, -126° and -210° C. It consisted of HF (6.3 mmol), UF₆ (9.58 mmol) and a trace of NF₃. Since the NF₄ HF₂ solution had taken up only about one third of the stoichiometric amount of UF₆, the recovered UF₆ was condensed back into the reactor. The mixture was stirred at 25° C. for 12 hours and the volatile material was pumped off again and separated. It consisted of HF (12.8 mmol), UF₆ (1.7 mmol) and a trace of NF₃. Continued pumping resulted in the evolution of only a small amount of UF₆, but no NF₃ or HF, thus indicating the absence of any unreacted NF₄ HF₂. The pale yellow solid residue (5.711 g, 88% yield) was shown by vibrational and ¹⁹ F NMR spectroscopy and elemental analysis to have the following composition (weight %): NF₄ UF₇, 97.47; NF₄ SbF₆, 1.50; CsSbF₆, 1.03. Anal. Calcd: NF₃, 15.34; U, 50.32; Sb, 0.90; Cs, 0.37. Found: NF₃, 15.31 U, 50.2; Sb, 0.90; Cs, 0.37.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described herein. 

What is claimed as new and desired to be secured by Letters Patent of the United States is:
 1. NF₄ UF₇.
 2. NF₄ WF₇.
 3. A process or preparing NF₄ UF₇ comprising the following steps in order:(1) placing NF₄ HF₂.nHF into a reaction vessel; (2) adding an excess of UF₆ to the reaction vessel; (3) allowing the UF₆ to react with the NF₄ HF₂.nHF; (4) pumping off the volatile products of the reaction; and (5) repeating steps (2) through (4) until the NF₄ HF₂.nHF is substantially converted to NF₄ UF₇.
 4. The process of claim 3 wherein n is from about 0.5 to about 10.0.
 5. The process of claim 3 wherein steps (2), (3), and (4) are performed at ambient temperature.
 6. A process for preparing NF₄ WF₇ comprising the following steps in order:(1) placing NF₄ HF₂.nHF into a reaction vessel; (2) adding an excess of WF₆ to the reaction vessel; (3) allowing the WF₆ to react with the NF₄ HF₂.nHF; (4) pumping off the volatile products of the reaction; and (5) repeating steps (2) through (4) until the NF₄ HF₂.nHF is substantially converted to NF₄ WF₇.
 7. The process of claim 6 wherein n is from about 0.5 to about 10.0.
 8. The process of claim 6 wherein steps (2), (3), and (4) are performed at ambient temperature. 